1. Field of the Invention
This invention relates to novel sodium-containing lanthanum, copper oxide compositions which are superconducting and to a process for making them.
2. References
Bednorz and Muller, Z. Phys. B64, 189 (1986), disclose a superconducting phase in the La-Ba-Cu-O system with a superconducting transition temperature of about 35 K. This disclosure was subsequently confirmed by a number of investigators [see, for example, Rao and Ganguly, Current Science, 56, 47 (1987), Chu et al., Science 235, 567 (1987), Chu et al., Phys. Rev. Lett. 58, 405 (1987), Cava et al., Phys. Rev. Lett. 58, 408 (1987), Bednorz et al., Europhys. Lett. 3, 379 (1987)]. The superconducting phase has been identified as the composition La.sub.1-x (Ba,Sr,Ca).sub.x O.sub.4-y with the tetragonal K.sub.2 NiF.sub.4 -type structure and with x typically about 0.15 and y indicating oxygen vacancies.
Over 400 scientific papers have appeared covering various aspects of these materials. Only three papers discuss attempts to incorporate an alkali metal into the structure and all three have used potassium. Ogita et al., Japn. J. Appl. Phys. 26, L415 (1987) disclose the preparation of (La.sub.0.95 K.sub.0.05).sub.2 CuO.sub.4-y by mixing prescribed amounts of La.sub.2 O.sub.3, CuO and K.sub.2 CO.sub.3, preheating the mixture at 700.degree. C. in air for 12 hours and sintering at 1000.degree. C. in air for 2 hours. The (La.sub.0.95 K.sub.0.05).sub.2 CuO.sub.4-y product was found to be of the orthorhombic K.sub.2 NiF.sub.4 type structure and was a semiconductor and not a superconductor down to liquid helium temperature.
Fine et al., "Chemistry of High-Temperature Superconductors", ACS Symposium Series 351, Edited by D. L. Nelson, M. S. Whittingham and T. F. George, American Chemical Society, Wash., D.C. (1987), Chapter 10, discuss the results of a study on the effect of nominal composition on superconductivity in La.sub.2 CuO.sub.4-y. They mixed the oxides of La.sub.2 O.sub.3 and CuO in appropriate amounts to form samples of the with nominal stoichiometries La.sub.2 CuO.sub.4-y and La.sub.1.9 CuO.sub.4-y respectively. Each mixture of starting materials was divided into three portions and 5% K.sub.2 CO.sub.3 and 10% K.sub.2 CO.sub.3 were added to the second and third portions of each mixture. A sample of nominal composition La.sub.1.9 K.sub.0.1 CuO.sub.7 was also prepared from appropriate amounts of La.sub.3 O.sub.3, CuO, and K.sub.2 CO.sub.3. The x-ray powder diffraction data indicated that all compositions were single phase or nearly single phase and the authors suggested that either the K.sub.2 O sublimed out of the sample or was present in a poorly crystalline form. The lattic parameters calculated for all seven samples were the same within two standard deviations and were consistent with the published values for the lattice parameters for La.sub.2 CuO.sub.4=y. The authors state that the implications are "that in all cases the bulk of the sample is stoichiometric, La.sub.2 CuO.sub.4-y, and that a solid solution of then type La.sub.2-x K.sub.x CuO.sub.4-y did not form".
Fine et al., Phys. Rev. B 36, 5716 (1987), report on the same study discussed in their ACS Symposium Series 351 chapter along with data on an eighth sample of nominal composition La.sub.1.8 CuO.sub.4-y and state the same conclusions.
There is no disclosure that substitution of any alkali metal into La.sub.2 CuO.sub.4 will result in a single phase composition exhibiting superconductivity above liquid helium temperatures and no disclosure of a composition of the formula La.sub.2-x A.sub.x CuO.sub.4-y where A is an alkali metal other than potassium.